1. Field of the Invention
The present invention relates to a process for the protection and lubrication of surfaces subjected to friction and wear, such as for instance magnetic recording media and accessories, including magnetic tape cassettes, by applying a protective coating comprising a fluorinated polyurethane.
More particularly, the present invention relates to the use of polyurethane resins containing polyoxyfluoroalkylene blocks to obtain coatings that protect such surfaces from friction and wear, when said surfaces are subjected to sliding, rolling, transverse forces or normal forces, or to movement involving either continuous or repetitive intermittent contact with other surfaces of a similar or different nature.
such friction problems occur in any device that employs magnetic recording media, and in particular, in audio and video cassettes or recording discs.
2. Description of the Prior Art
In order to minimize the drawbacks resulting from friction and wear, it is known to use neutral perfluoropolyethers as lubricants for the surfaces of magnetic discs and tapes for audio and video recordings.
Perfluoropolyethers suitable for this purpose have been sold under the trademarks "FOMBLIN" (Montedison) and "KRYTOX" (duPont), and they are described, for instance, in U.S. Pat. Nos. 3,242,218, 3,665,041 and 3,715,378. However, these perfluoropolyethers do not provide long-term protection because of insufficient adhesion to the substrate (carrier surface).
Some improvements in the lubricating properties of perfluoropolyethers have been achieved as described in U.S.
Pat. Nos. 4,268,556 and 4,267,238, by introducing into the perfluoropolyether chain polar and chemically reactive end groups such as --CH.sub.2 OH, --COOR, --CONH--R and --CO--CF.sub.3 in the thought that these would provide improved adhesion of the perfluoropolyether compound to the surface to be lubricated. However, the improvement in adhesion is frequently unsatisfactory, and will depend upon the particular substrate onto which the perfluoropolyether products are applied. In the case of polymeric substrates or metal oxide particles dispersed in polymeric matrixes, these lubricants have not been sufficiently compatible with the substrates, so that their adhesion and resulting protective action was of short-term duration.
Furthermore, such modified perfluoropolyethers exhibit the disadvantage of a high reactivity of the end groups, some of which, for instance, easily hydrolyze to form acid groups which react with the materials on which they are deposited, thereby adversely altering the properties of said materials.
As regards manufactured articles containing members subjected to friction and wear, coatings made of polytetrafluoroethylene (PTFE) are also known. The PTFE protective layers, although quite wear-resistant, exhibit drawbacks both with regard to the particular surface to be protected, since PTFE layers are of only limited flexibilty, as well as regards the type of application technique that can be used for said protective layer. Indeed, since the PTFE is insoluble in organic solvents, it must be applied as a polymer dispersion. By such technique, however, it is impossible to obtain sufficiently thin, homogeneous layers of polymer. Thick layers create problems, for instance, a lower signal level due to the increased distance between the reading/recording head and the magnetic tape.
European published application Nos. 165,649 and 165,650 describe the use of fluoropolyethers containing special end groups suitable for improving adhesion of the coating to the surface of the substrate.
Such fluoropolyethers exhibit good lubricating properties and may be conveniently used in the lubrication of both magnetic recording media as well as for accessories of audio and video recording cassettes. The best results with said fluoropolyethers are, however, primarily achieved only when applied to metal surfaces.
Thus, there has been a need for a process which is useful for the protection of surfaces of either a polymeric or metallic nature, these surfaces being subjected to friction and wear, by a protective coating capable of providing long-lasting protection and lubrication of said surfaces.
Likewise, it is desirable that the protective coating be capable of being spread onto the surface to be protected both in the form of very thin layers, in the range of 50-1,000 .ANG., so that there will be no alteration in the response signal of the magnetic media, as well as in the form of thicker layers, in the range of 1,000-50,000 .ANG., for accessory members where no signal response problems exist.